Injection molding machine and back pressure control method of the same

ABSTRACT

An injection molding machine is provided which uses servo motors (M1, M2) as driving sources and in which a preset back pressure is correctly transmitted to a resin. The torque limit values and rotating directions of an injection servo motor in respective stages is set in accordance with the relationship in magnitude between the preset back pressure and the resistance during a transmission mechanism of the respective stages of metering. A servo circuit of an injection servo motor is torque-limited with a preset torque limit value, and the injection servo motor is driven in a preset rotating direction.

BACKGROUND OF THE INVENTION

The present invention relates to an injection molding machine in which aservo motor for driving an injection mechanism controls a back pressureapplied to a molten resin during metering and a back pressure controlmethod of the same. More particularly, the present invention is aninjection molding machine which can correctly control the value of theback pressure for a wide range and a back pressure control method of thesame.

In the metering process of an injection molding machine, a resin as amolding material is plasticized to a molten state in a heating cylinderby rotation of a screw, and the molten resin is then stocked in thedistal end portion of the heating cylinder. The screw is moved backwardby the pressure of the molten resin. When the screw is moved backward toa preset metering position, the rotation of the screw is stopped, andthe metering process is ended.

Conventionally, in order to adjust the melting and metering effect forthe resin, the back pressure is controlled during metering. For example,in an injection molding machine of a type in which an injectionmechanism is driven by hydraulic pressure, the back pressure is alsocontrolled by the hydraulic pressure. Recently, in a molding machine ofa type in which an injection mechanism is driven by a servo motor, amethod has been developed to perform back pressure control by applying atorque limit to the servo motor (refer to Japanese Patent ApplicationNo. 60-88911).

In an injection molding machine using a servo motor, the injection servomotor is driven so that the screw is kept at the present position duringmetering. However, as the screw rotates, the resin pressure is increasedto move the screw backward, thereby causing a positional error. In thiscase, a drive command is output to the servo motor to return the screwto the initial position. However, since the drive current of the servomotor is limited by a torque limit value, corresponding to the presetback pressure, a force exceeding the preset back pressure is not appliedto the resin.

In order to apparently eliminate the positional error caused as a resultof this, a numerical controller detects the value of an error register,and subtracts the detected value from the error register, therebysetting the register value to zero, in other words, following up thevalue of the error register. In practice, however, a delay occurs in thetime required for the numerical controller to complete follow-up of thevalue of the error register. Therefore, the screw is moved backwardduring this delay time. Even if the value of the error register isfollowed up, the error register is not actually set to zero. As aresult, a motor drive current command corresponding to the torque limitvalue is always output, and the motor output torque is in accordancewith the torque limit value. When the screw is moved backward to reachthe preset metering position, the rotation of the screw is stopped,thereby completing the metering process.

However, with a conventional servo-type back pressure control system,back pressure control cannot be correctly performed because of thereasons to be described later.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an injection moldingmachine which can apply a preset back pressure to a resin and a backpressure control method of the same.

The present invention is based on the recognition that the factors whichvary the back pressure in the servo motor-type back pressure controlsystem are in a transmission mechanism between the servo motor and thescrew. More specifically, the transmission mechanism of the backpressure control system of this type includes, e.g., a ball screw andnut to convert rotational movement of the servo motor into linearmovement of the screw. The above transmission mechanism has a morecomplex structure than a transmission mechanism of a hydraulic-typecontrol system. The back pressure control transmission mechanism havingthe above arrangement generates a resistance (to be merely referred toas a frictional force hereinafter) such as a frictional force that acts,when the screw is moved backward by the resin pressure, in a directionto interfere with the backward movement. The generated frictional forcevaries the back pressure which is typically applied to the resin.

In order to achieve the above object, according to the presentinvention, an injection molding machine controls by a numericalcontroller a servo motor for rotating a screw and an injection servomotor for driving the screw in an axial direction through a transmissionmechanism, thus performing injection which thereby performs metering.Back pressure control is performed by applying a torque limit to a servocircuit for driving the injection servo motor, wherein a torque limitvalue of each stage and a rotating direction of the injection servomotor in each stage are set in accordance with a relationship inmagnitude between a preset back pressure of the corresponding stage ofthe metering process and a resistance, including a frictional force,that is generated in the transmission mechanism. The servo circuit ofthe injection servo motor is torque-limited with the preset torquelimited value in each stage, and the injection servo motor is driven inthe preset rotating direction.

As described above, according to the present invention, a torque limit,which is obtained by correcting a preset back pressure with a resistancegenerated by the transmission mechanism between the injection servomotor and the screw, is applied to the injection motor. Thus, the servomotor is driven in such a direction as to move the screw forward orbackward in accordance with the relationship in magnitude between thepreset back pressure and the resistance of the transmission mechanism.Therefore, the preset back pressure can be correctly applied to theresin.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing a back pressure control system of aninjection molding machine according to an embodiment of the presentinvention;

FIG. 2 is a flow chart of a control program for performing processing toset the torque limit value for metering conditions and the rotatingdirection of an injection servo motor;

FIG. 3 is a table for setting the metering conditions; and

FIG. 4 is an operation processing flow chart for metering.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a back pressure control system of an injection moldingmachine according to an embodiment of the present invention. Referringto FIG. 1, reference numeral 1 denotes an operational circuit portion ofa numerical controller of the injection molding machine. The circuitportion 1 is connected to a servo circuit 2 for driving a servo motorM1, which rotates a screw (not shown), and to a servo circuit 3 fordriving an injection servo motor M2, which drives the screw in an axialdirection and supplies a back pressure during injection and metering.The servo motors M1 and M2 are provided with pulse encoders P1 and P2,respectively, for detecting their positions and speeds. A D/A converter4, for converting a torque limit command supplied from the operationalcircuit portion 1 into an analog voltage, is also provided.

The operational circuit portion 1 of the numerical controller 1 has amicrocomputer 10, a memory 11, a nonvolatile RAM 12, a manual data inputunit (to be referred to as a CRT/MDi hereinafter) 13 with a CRT display,a pulse distributor 14, and an input/output circuit 15. These circuitcomponents are connected via a bus 16. The memory 11 includes a ROM forstoring a control program for controlling the injection molding machine,a RAM for temporarily storing data, and so on. The nonvolatile RAM 12stores various preset values to be described later. The pulsedistributor 14 drives the servo motors M1 and M2.

The servo circuit 3 has an error register 31 and a D/A converter 32. Theerror register 31 stores a difference between the movement commandsupplied from the operational circuit portion 1 and the movement amountof the servo motor M2, supplied from the pulse encoder P2. The D/Aconverter 32 converts the value of the error register 31 into an analogsignal and outputs a velocity command. The servo circuit 3 also includesan F/V converter 33 and an error amplifier 34. The F/V converter 33converts the signal, representing the present speed of the motor M2 andsupplied from the pulse encoder P2, into a voltage. The error amplifier34 compares a value, obtained by adding the velocity command suppliedfrom the D/A converter 32 to an offset voltage output from theoperational circuit portion 1 (through the input/output circuit 15 to bedescribed later), with a voltaage output from the converter 33, andamplifies the difference therebetween. The amplified difference isoutput as a drive current command, i.e., a torque command, to a torquelimit means 35. The means 35 receives the torque limit command outputfrom the input/output circuit 15 of the operational circuit portion 1,through the D/A converter 4, and clamps the output from the erroramplifier 34 with a value corresponding to the torque limit command andoutputs the clamped value. The servo circuit 3 also has an erroramplifier 36 and a power amplifier 37. The error amplifier 36 comparesthe drive current command output through the error amplifier 34 and thetorque limit means 35 with a signal supplied from a current detector 38,for detecting the drive current of the servo motor M2, and amplifies thedifference therebetween. The power amplifier 37 is operated by theoutput from the error amplifier 36.

With the above arrangement, first, the operator sets various conditionsfor the metering process via the CRT/MDi 13. More specifically, a screwrotation speed SCi, a preset back pressure BSi, and a screw position SWiof a switch point, or a terminal at which switching to the next stage isperformed, of the respective stages i (i=1, 2, . . . ) of the meteringprocess are set in the nonvolatile RAM 12.

Even if the injection servo motor M2 is driven during metering byapplying a torque limit with a value corresponding to the preset backpressure BSi, because of the influence of a frictional force R, or thelike, of a transmission mechanism (not shown) that transmits the torqueof the servo motor M2 to the screw, the force actually applied to theresin is not equal to the preset back pressure. More specifically, thefrictional force R acts in a direction to interfere with the backwardmovement of the screw caused by the resin pressure, and the forceactually applied to the resin is a value obtained by adding thefrictional force R to the output torque BPi of the injection motor M2,as indicated in following equation (1):

    Force actually applied to resin=Output torque BPi of motor M2+frictional force R                                                   (1)

In order to obtain a force applied to the resin equal to the preset backpressure BSi, the output torque BPi of the motor may be set to a valueobtained by subtracting the frictional force R from the preset backpressure BSi, as indicated in equation (2):

    BPi=force BSi to be applied to resin-R                     (2)

As is understood from equation (2), when the preset back pressure BSi islarger than the frictional force R, the output torque BPi of the motorM2 is positive. Therefore, the motor may be driven in the forwarddirection while applying a torque limit, so that the screw is driven inthe forward direction. However, when the preset back pressure BSi issmaller than the frictional force R, the output torque BPi of the motoris negative, and the motor M2 must be driven in the reverse direction soas to drive the screw in the backward direction. More specifically, whenthe prest back pressure BSi is small and the frictional force R islarge, in order to set the force actually applied to the resin to thepreset back pressure BSi, the servo motor M2 drives the screw in thebackward direction and the motor M2 generates an output torque of R-BSi.Then, since the output torque BPi=(R-BSi), of the motor acts in adirection to drive the screw backward, and the frictional force R actsin a direction to interfere with the backward movement of the screw, thecomposite force of them, i.e., a force actually applied to the resin isequal to the back pressure BSi, as indicated in equation (3) below:

    Force actually applied to resin=R-(R-BSi)=BSi              (3)

Therefore, when the servo motor M2 is driven by applying a torque limitto control the back pressure, the drive direction of the servo motor M2must be considered by taking into account the relationship in magnitudeof the preset back pressure and the frictional force R. Some numericalcontrollers have an offset function to correct the offset voltage of aD/A converter and so forth. This embodiment utilizes this offsetfunction in order to control the drive direction of the injection servomotor M2. For this purpose, when the metering conditions are to be set,the CPU 10 performs the processing shown in FIG. 2 so that thenonvolatile RAM 12 stores the metering conditions as shown in FIG. 3.

When the screw rotation speed SCi and the switching point screw positionSWi of the respective stages i in the metering process are set by theCRT/MDi 13, the CPU 10 directly stores the data in the table T of thenonvolatile RAM 12, as shown in FIG. 3. Subsequently, every time eachback pressure BSi is set, the CPU 10 performs the processing shown inFIG. 2, and causes the nonvolatile RAM 12 to store the torque limitvalue BPi (the output torque of the servo motor M2) and the drivedirection SGNi of the servo motor M2.

More specifically, when a back pressure BSi is set, the CPU 10 detectsthe setting (step S1), subtracts the frictional force R of thetransmission mechanism of the injection molding machine from the presetback pressure BSi, and determines if the subtraction result is positiveor negative (step S2).

If YES in step S2, the preset back pressure BSi is larger than thefrictional force R, and the screw is to be driven in the forwarddirection. The value obtained by subtracting the frictional force R fromthe preset back pressure BSi is stored in the RAM 12 as a torque limitvalue BPi which is an output torque of the servo motor M2 (step S3), andthe direction (forward) "0" to move the screw forward is stored as arotating drive direction SGNi of the servo motor M2 (step S4).

If NO in step S2, the preset back pressure BSi is smaller than thefrictional force R. Therefore, as described above, a value obtained bysubtracting the preset back pressure BSi from the frictional force R isstored as the torque limit value BPi, and the direction (reverse) "1" tomove the screw backward is stored as a rotating drive direction SGNi ofthe servo motor M2 (steps S5 and S6).

In this manner, the screw rotation speed SCi, the torque limit valueBPi, the switching screw position SWi, and the driving direction SGNi ofthe servo motor M2, of the respective stages i of the metering processare preset and stored in the table T of the nonvolatile RAM 12.

In this embodiment, the back pressure BSi is preset. However, thefrictional force R can be subtracted in advance from the back pressureBSi, the absolute value of the subtraction result may be set as thetorque limit value BPi, and the rotating drive direction SGNi may bemanually set.

After the various data are set, when the injection molding machine isactuated to start the metering process, the metering process shown inFIG. 4 is started. The CPU 10 sets a counter i to 1 (step S7), and ascrew rotation speed SC1 of the first stage is read out from the table Tof the nonvolatile RAM 12. If the data is correctly set, the CPU 10determines that the screw rotation speed SC1 is not "0" (step S8). Thescrew rotation speed SC1 read out from the table T is output to theservo circuit 2 of the screw servo motor M1. The servo motor M2 thendrives the servo motor M1 with the preset screw rotation speed SC1. TheCPU 10 reads the torque limit value BP1 of the first stage from thetable T, and BP1 is output to the D/A converter 4 through theinput/output circuit 15 (step S9). The torque limit value BP1 isconverted into an analog voltage by the converter 4 and is supplied tothe torque limit means 35. The means 35 clamps the output of the erroramplifier 32 with the torque limit value BP1.

Subsequently, the CPU 10 reads a drive direction SGN1 of the servo motorM2 of the first stage from the table T, and determines if the directionSGN1 is "1" (step S10). If NO in step S10, an offset voltage OF is notoutput. If YES in step S10, an offset voltage OF is output and added tothe output from the D/A converter 32 (steps S11 and S12).

When the direction SGN1 is not "1", that is, when the preset backpressure BS1 is larger than the frictional force R, the offset commandOF is not output. In this case, since no movement command has been inputto the error register 31, the servo motor M2 remains stopped. Then, whenthe servo motor M1 is driven, the screw is rotated, the resin is melted,the molten pressure becomes high, and the screw is pushed backward. As aresult, the servo motor M2 is rotated in the reverse direction and apulse train, representing reverse rotation of the servo motor (backwardmovement of the screw), is output from the pulse encoder P2 of the servomotor M2 and added to the error register 31. The value of the errorregister 31 is converted into a voltage by the D/A converter 32,amplified by the error amplifier 34, and output to the torque limitmeans 35. This output is clamped by the torque limit means 35.Therefore, no drive current command (torque command) exceeding thetorque limit value BP1 set in the torque limit value 35 is output.

The servo motor M2 is driven by a drive current corresponding to thetorque limit value BP1, through the error amplifier 36 and the poweramplifier 37, so that the error register 31 becomes "0", i.e., the servomotor M2 is returned to the initial position. As a result, the resin ispressed by the screw with a torque corresponding to the torque limitvalue BP1, i.e., with the preset back pressure BS1. When the moltenpressure of the resin exceeds the preset back pressure BS1, the screw ismoved backward because of the differential pressure between the moltenresin pressure and the preset back pressure.

Subsequently, the CPU 10 reads the value of the error register 31through the input/output circuit 15, adds this value to a present valueregister SP (FIG. 4) for storing the present position of the screw (stepS13), inverts the sign of the read value of the error register 31, addsthe inverted value to the error register 31, and updates the register(step S14). In this manner, the value of the error register 31 is set to"0" to maintain the servo motor M2, i.e., the screw position, at thepresent position. The screw is moved backward and the servo motor M2 isrotated in the reverse direction even while the CPU 10 performs thisprocessing. Therefore, the value of the error register never becomes"0".

As a result, a velocity command in a direction to move the screw forwardis output from the D/A converter 32, added with the output from the F/Vconverter 33 (since the motor is rotated in the reverse direction, theoutput from the F/V converter 33 becomes negative, and addition isperformed), amplified by the error amplifier 34, and output.

More specifically, since the drive current comman (torque command) isclamped by the torque limit means 35 with the preset torque limit valueBP1, the servo motor M2 is driven in the forward direction with a torquecorresponding to the torque limit value. The CPU 10 then determineswhether the value of the present value register SP has reached theswitch point SW1 from the first to second stage (step S15). If NO instep S15, processing of steps S13 to S15 is repeated. When the value ofthe present value register SP (i.e., the screw position) reaches theswitching screw position SW1, the count i of the counter is incrementedby "1" (step S16), and processing following step S8 is performed for thesecond stage of the metering process.

If YES in step S10, i.e., when the preset back pressure BSi is smallerthan the frictional force R, the CPU 10 outputs an offset voltage of adirection (a direction to rotate the motor in the reverse direction) tomove the screw backward (step S11). The offset voltage is amplified bythe error amplifier 34 and clamped by the torque limit means 35. Then,the drive current command is clamped at the preset torque limit valueBPi and the output torque of the servo motor M2 is limited, therebydriving the servo motor M2 in a direction to move the screw backward.Therefore, as indicated in equation (3), the force to be applied to theresin equals the preset back pressure BSi, and processing similar to thesteps after step S13 is performed. In this manner, when all stages ofthe metering process are completed and the screw rotation speed SCi readout from the table T is determined to be SCn, i.e., "0" (step S8), themetering process is completed.

Although certain preferred embodiments have been shown and described, itshould be understood that many changes and modifications may be madetherein without departing from the scope of the appended claims.

I claim:
 1. An injection molding machine including a servo motor forrotating a screw and an injection servo motor for driving the screw inan axial direction through a transmission mechanism and performinginjection, the sevo motor and the injection servo motor being driven byservo circuits respectively controlled by a numerical controller forperforming multi-stage metering control and a back pressure control,said back pressure control comprising the steps of:(a) setting a torquelimit value for each of the metering stages and a rotating direction ofthe injection servo motor for each of the metering stages in accordancewith a relationship in magnitude between a preset back pressure for anassociated one of the metering stages and a value representative ofmechanical energy loss; (b) applying a torque limit to the servo circuitof the injection servo motor with the preset torque limit value for anassociated one of the metering stages; and (c) driving the injectionservo motor in the preset rotating direction.
 2. The method ofcontrolling back pressure according to claim 1, wherein said step (a)includes setting the torque limit value for an associated one of themetering stages at a value obtained by subtracting the resistance of thetransmission mechanism from the preset back pressure for an associatedone of the metering stages.
 3. The method of controlling back pressureaccording to claim 1, wherein the rotating direction of the injectionservo motor for each stage is set in a direction to drive the screw inan injection direction when the preset back pressure for an associatedone of the metering stages is larger than the resistance of thetransmission mechanism, and the rotation direction is set in a directionto drive the screw opposite to the injection direction when the presetback pressure of the corresponding stage is smaller than the resistanceof the transmission mechanism.
 4. The method of controlling backpressure according to claim 2, wherein the rotating direction of theinjection servo motor for each stage is set in a direction to drive thescrew in an injection direction when the preset back pressure for anassociated one of the metering stages is larger than the resistance ofthe transmission mechanism, and the rotation direction is set in adirection to drive the screw opposite to the injection direction whenthe preset back pressure of the corresponding stage is smaller than theresistance of the transmission mechanism.
 5. An injection moldingmachine where a servo motor for rotating a screw and an injection servomotor for driving the screw in an axial direction through a transmissionmechanism and performing injection, are driven by servo circuitsrespectively controlled by a numerical controller for performing ametering operation in a multi-stage fashion, the metering stagesstarting at corresponding axial positions of the screw and ending atdifferent axial positions of the screw, respectively, one metering stageending and another adjacent metering stage starting when the screwreaches an associated switching point in its axial position, thetransmission mechanism having a plurality of moving elements betweenwhich a mechanical energy loss, including a loss resulting from africtional force generated between the elements, is produced when thetransmission mechanism operates, said injection molding machinecomprising:memory means for storing torque limit values and rotatingdirections of the injection servo motor and the switching points for themetering stages, each of the torque limit values obtained by subtractinga value representative of the mechanical energy loss from a preset backpressure for an associated one of the metering stages, each of therotating directions being determined in accordance with a relationshipin magnitude between the preset back pressure for an associated one ofthe metering stages and the value representative of the mechanicalenergy loss; means for detecting the screw position; switching means forcomparing the detected screw position with each of the stored switchingpoints, and for reading out from said memory means and outputting thetorque limit value and the rotating direction for an associated one ofthe metering stages which is determined by the comparison; torque limitmeans for applying a torque limit to the servo circuit of the injectionservo motor, with the torque limit value output from said switchingmeans; and means for driving said injection servo motor in the rotatingdirection output from said switching means.
 6. The injection moldingmachine of claim 5, wherein said rotating drive means includes offsetoutput means for selectively outputting an offset voltage to said servocircuit of the injection servo motor in accordance with the readoutrotating direction.
 7. The injection molding machine according to claim5, further comprising means coupled to said memory means for maintainingthe screw position of the servo motor at a preset position.
 8. Theinjection molding machine according to claim 5, wherein said memorymeans stores screw rotation speeds for the metering stages, saidswitching means reading out the screw rotation speed for a correspondingone of the metering stages from said memory means in accordance with thecomparison result, and said screw rotation control means drives theservo motor for rotating the screw at the screw rotation speed read outthrough said switching means.
 9. The injection molding machine accordingto claim 6, further comprising means coupled to said memory means formaintaining the screw position of the servo motor at a present position.10. The injection molding machine according to claim 6, wherein saidmemory means stores screw rotation speeds for the metering stages, saidswitching means reads out the screw rotation speed for a correspondingone of the metering stages from said memory means in accordance with thecomparison result, and said screw rotation control means drives theservo motor for rotating the screw at the screw rotation speed read outthrough said switching means.